Electronic image carriers – Digital weaknesses

Silver film can be pushed, i.e., underexposed and forcibly developed, in order, for example, to be able to take hand-held photographs with too low a film speed in too little light. Similarly, one can adjust the sensitivity of the semiconductor elements either up or down. Similar to its analog counterpart, adjusting the sensitivity here results in a coarsened image output. The underexposure causes the sensors to receive less light than necessary, necessitating the amplification of their output signal. Inevitably, the electrical noise (the error potential superimposed on the signal) is also amplified. However, since the image was originally underexposed but the resulting weak signal contained the normal amount of noise, the unwanted snow-like artifacts visibly appear after amplification and overlay the image. Let’s be clear: The noise is always present, even at lower ISO values, but it is much weaker compared to the incoming signal and therefore hardly noticeable.

Digital image carriers do not exhibit a Schwarzschild effekt similar to silver film, but they often react to long exposure times with so-called hot pixels. This is caused by the uncontrolled discharge of a CCD or CMOS cell when it exceeds its charge limit and manifests itself as white, red or blue spurious pixels that are particularly annoying in dark parts of the image. To prevent this, some manufacturers limit the longest possible exposure time or counteract it with correspondingly trickily programmed firmware. If you want to go further into the long exposure range than the manufacturer intended, you can reduce the interference potentials by cooling the camera, as is done in the field of astrophotography, or by removing them afterwards with software like the „Hotpixels Eliminator“. By the way, the photoreceptors in our eyes also produce such unwanted discharges under the influence of thermal radiation. – If you have been lying in bed at night for some time and are fully scotopically adapted, see if you notice the tiny bright dots.

Blooming is a problem unique to CCD chips. Triggered by particularly bright subject areas, individual sensor areas are overexposed and can no longer control their voltage. Their charges virtually „spill over“ to adjacent areas of the chip. Overexposure causes a sudden loss of contrast in these bright parts of the image, resulting in an almost monochrome white area. Light sources often exhibit stripes or halos. Highly reflective surfaces, such as chrome or glass, are particularly susceptible to blooming. There are various hardware-based techniques to minimize the blooming effect. Both horizontal and vertical antiblooming techniques create a „gully“ adjacent to a pixel, allowing overflowing electrons to escape. Simple construction and effective impact are the advantages; however, the loss of space in the light-sensitive sensor area results in a reduction in sensitivity. Clocked antiblooming uses the property of electrons to recombine with „holes“ in the crystal lattice. We use special clocking to constantly replenish the supply of holes. The disadvantage, however, is that the pixels‘ capacity decreases, and the process is technically complicated.

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